Doped hydrogen storage material

ABSTRACT

A doped hydrogen storage material according to the general formula: 
       Mg x  B y  M z  H n    
     wherein:
         (i) the ratio of x/y is in the range of from 0.15 to 1.5;   (ii) z is in the range of from 0.005 to 0.35;   (iii) x+y+z equals 1;   (iv) M=is one or more metals selected from the group of selected Sc,       

       Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn;         (v) n is no more than 4y; and   wherein x/y does not equal 0.5 and at least part of the doped hydrogen storage material is amorphous.       
     The doped hydrogen storage materials are used for storing hydrogen, and also disclosed is a method for reversibly desorbing and/or absorbing hydrogen.

FIELD OF THE INVENTION

The invention provides a doped hydrogen storage material, the use ofsuch material for storing hydrogen and a method for reversibly desorbingand/or absorbing hydrogen using a doped hydrogen storage materialaccording to the invention.

BACKGROUND OF THE INVENTION

The storage of hydrogen in the form of metal hydrides has gained a lotof attention in the recent years. The storage of hydrogen in metalhydrides is based on chemisorption, i.e. no molecular hydrogen (H₂) isstored but the hydrogen reacts with the metal to form metal hydrides.Storage of hydrogen in the from of metal hydrides has the advantage overstorage of hydrogen in for instance liquid or compressed state in thatit does not require the use of low temperatures or excessive pressures.

In U.S. Pat. No. 6,106,801 it is disclosed that Ti-doped NaAlH₄compositions can be used to store hydrogen. U.S. Pat. No. 6,106,801discloses that by doping NaAlH₄ with Ti the hydrogen desorptiontemperature decreases from approximately 200° C. to 140° C. However,Ti-doped NaAlH₄ may comprise hydrogen atoms up to a maximum hydrogenstorage density per weight of storage material of approximately 5 wt %.

It has been proposed by Chlopek et al. (J. Mater. Chem.,2007,17,3496-3503) that a suitable alternative would be the use ofmagnesium tetrahydroborate, i.e. Mg(BH₄)₂. This hydride may comprise upto 14.9 wt % of hydrogen, based on the weight of the hydride. However,the onset temperature of hydrogen desorption is high, typicallytemperatures above 290° C. are required before hydrogen is released fromthe hydride.

Li et al.(Li et al., Dehydriding and rehydriding processes ofwell-crystallised Mg(BH₄)₂ accompanying with formation of intermediateproducts, Acta Mater (2008) doi10.1016/j.actamat. 2007.11.023) show thatwell-crystallized Mg(BH₄)₂ may be dehydrided. In a second step thedehydrided Mg(BH₄)₂ is rehydrided by subjecting the dehydridedwell-crystallized Mg(BH₄)₂ to hydrogen at a temperature of 543 K and apressure of 40 MPa for a time period of 48 hours. In a third,dehydriding, step, 6.1 mass % of hydrogen could be obtained from thematerial, which was rehydrided in the second step. Of the 6.1 mass %,3.9 mass % was attributed to the formation of MgH₂ during rehydriding inthe second step. Disadvantage of the process of Li et al. is that thisonly modest rehydriding takes place under severe pressure andtemperature conditions for prolonged times. Furthermore, it is undesiredto form MgH₂ during rehydriding. MgH₂ has a much lower hydrogen storagecapacity than the well-crystallized Mg(BH₄)₂.

There is still a need in the art for a hydrogen storage material thatallows a reversible storage of hydrogen at low hydrogen uptake andrelease temperatures and mild rehydriding conditions.

SUMMARY OF THE INVENTION

It has now been found that Mg and B comprising hydrogen storagematerials doped with a transition material may be prepared, which may bereversibly dehydrided and rehydrided under mild rehydriding conditions.

Accordingly, the present invention provides a doped hydrogen storagematerial according to the general formula:

Mg_(x) B_(y) M_(z) H_(n)

wherein:

-   -   (i) the ratio of x/y is in the range of from 0.15 to 1.5;    -   (ii) z is in the range of from 0.005 to 0.35;    -   (iii) x+y+z equals 1;    -   (iv) M=is one or more metals selected from the group of selected        Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn;    -   (v) n is no more than 4y; and

wherein x/y does not equal 0.5 and at least part of the doped hydrogenstorage material is amorphous.

Reference herein to doping is to the addition of an amount of anothermaterial.

The doped hydrogen storage material according to the invention shows anonset-temperature of hydrogen desorption of the corresponding hydrideswhich is significantly lowered compared to Mg(BH₄)₂.

Reference herein to the onset-temperature of hydrogen desorption is tothe lowest temperature at which hydrogen desorption is observed. In afurther aspect, the invention relates to the use of the doped hydrogenstorage material according to the invention to store hydrogen.

In an even further aspect, the invention relates to a method forreversibly desorbing and/or absorbing hydrogen using a doped hydrogenstorage material according to the invention, comprising:

-   a) dehydriding the doped hydrogen storage material by desorbing    hydrogen from the doped hydrogen storage material to obtain hydrogen    gas and a partially dehydrided doped hydrogen storage material,    whereby the obtained partially dehydrided doped hydrogen storage    material comprises at least 10 atomic %, in particular at least 30    atomic %, more in particular at least 50 atomic % of the maximum    amount of atomic hydrogen which can be stored in the doped hydrogen    storage material; and-   b) hydriding the partially dehydrided doped hydrogen storage    material by contacting the partially dehydrided doped hydrogen    storage material with a hydrogen-comprising gas to reversibly store    hydrogen and to obtain an at least partially rehydrided doped    hydrogen storage material.

The doped hydrogen storage material according to the present inventionprovides high storage capacity for hydrogen while allowing to retrievethe hydrogen from the storage material relatively low temperatures.

Reference herein to dehydriding is to desorption of hydrogen from thehydrogen storage material. Reference to hydriding or rehydriding is toabsorption of hydrogen in the hydrogen storage material.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a doped hydrogen storage materialcomprising Mg and B and having the following general formula:

Mg_(x) B_(y) M_(z) H_(n)   (1)

The doped hydrogen storage material according to the invention andrepresented by formula (1) comprises Mg and B in a molar ratio (x/y) ofin the range of from 0.15 to 1.5, preferably of from 0.15 to 0.7,whereby the molar ratio of Mg and B is not 0.5. The preferred molarratios of Mg and B provide the lowest hydrogen desorptiononset-temperatures. More preferably, the doped hydrogen storage materialaccording to the invention and represented by formula (1) comprises Mgand B in a molar ratio (x/y), which is equal to 0.48 or higher, evenmore preferably in the range of from 0.48 to 0.70, while the molar ratioof Mg and B (x/y) is not 0.5. Such molar ratios of Mg and B may providehigh hydrogen storage capacity. Without wishing to be bound to aparticular theory, it is presently believed that by providing acomposition comprising Mg and B in a ratio other than the stoichiometricmolar ratio of Mg and B, i.e. a molar ratio of Mg to B of 0.5, inducesthe formation of favourable compositions other than Mg(BH₄)₂. It isfurther believed that this effect is noticeable even when the deviationfrom the stoichiometric composition is small, for instance the molarratio of Mg and B (x/y) is at least outside the range of from 0.49 to0.51.

The doped hydrogen storage material according to the invention is atleast partly amorphous or may be completely amorphous. Reference hereinto amorphous is to a non-crystalline structure, i.e. at least part ofthe material has a non-crystalline structure. Preferably at least 5%,more preferably at least 50%, even more preferably at least 90%, stilleven more preferably 95% of the hydrogen storage material is amorphous.Reference herein to a non-crystalline structure is to a structure forwhich in an X-ray Diffraction (XRD) analysis no crystalline peaks can beidentified. As described above and without wishing to be bound to aparticular theory, it is presently believed that the amorphous ornon-crystalline structure is preferred due to its improved diffusionproperties compared to the crystalline material.

The doped hydrogen storage material according to the present inventioncomprise besides Mg and B, a metal dopant represented in formula (1) asM. M is one or more transition metals selected from the group of Sc, Ti,V, Cr, Mn, Fe, Co, Ni, Cu and Zn. It has been found that the metaldopant catalyses the (re)hydriding and dehydriding processes in thehydrogen storage material. In addition it was found that the dopant mayinduce a significant decrease of the hydrogen desorption temperature.Preferably, the dopant is Ti and/or Ni, whereby Ti is more preferred dueto its lower atomic weight.

Typically, the hydrogen desorption properties of a doped hydrogenstorage material do not depend on the amount of dopant added, provided aminimum amount of dopant is present. A factor limiting the amount ofadded dopant is the increased weight of the hydrogen storage materialand resulting lower hydrogen storage density per weight of storagematerial.

The amount of dopant present in the composition according to formula (1)is given by z and z is in the range of from 0.005 to 0.35. It was foundthat in the case of the doped hydrogen storage material according to thepresent invention the hydrogen desorption behaviour, i.e. the hydrogendesorption temperature, may be optimized by choosing the amount ofdopant in the doped hydrogen storage material. Such preferred amounts ofdopant are obtained by providing a material having a compositionaccording to formula (1) wherein z is preferably in the range of from0.005 to 0.1, more preferably of from 0.02 to 0.07.

In formula (1), x may be in the range of from 0.2 to 0.6 and y may be inthe range of from 0.4 to 0.85. Preferably, more than 50% of the metalatoms are B atoms, i.e. y>0.5, more preferably, y is in the range offrom 0.5 to 0.85. Compositions comprising increased amounts of B maystore higher quantities of hydrogen due to its higher hydrogen storagedensity per weight of storage material. In formula (1), the sum of x, yand z must be 1.

The hydrogen storage material according to formula (1) may also comprisehydrogen. It will be appreciated that the amount of hydrogen depends onwhether the hydrogen storage material is fully hydrided or partiallyhydrided. The maximum amount of hydrogen that may be stored in thehydrogen storage material is related to the amount of B and to a lesserextent Mg present in the material and on the state of the hydrogen atomsin the hydrided material. Without wishing to be bound to a particulartheory, it is presently believed that each B atom in the hydrogenstorage material may bind up to 4 hydrogen atoms. Therefore n is in therange of from 0 to 4y. When n is 0, no hydrogen is present in thehydrogen storage material. Typically, n is 0 only for doped hydrogenstorage materials as prepared in the absence of hydrogen. After thefreshly made hydrogen-free doped hydrogen storage material is contactedwith hydrogen for the first time, the material will always contain somehydrogen and n will not be 0.

It will be appreciated that trace amounts of other metal atoms mayalways be present in the doped hydrogen storage material according tothe invention, however such trace amounts of other metals do not affectthe hydrogen storage behaviour of the doped hydrogen storage material.

Such traces of other metal atoms may be due to for instance impuritiesin the separate components or impurities introduced during thepreparation of the doped hydrogen storage materials. Preferably, the Mgand B together with the dopant make up at least 99 atomic %, morepreferably at least 99.99 atomic % of the metal atoms present in thedoped hydrogen storage material. More preferably, the doped hydrogenstorage material comprises no metal atoms other than Mg, B, Sc, Ti, V,Cr, Mn, Fe, Co, Ni, Cu and Zn.

The doped hydrogen storage material according to the invention may beprepared by intimately mixing metallic Mg and B or an inter-metalliccompound of Mg and B with the dopant. Optionally, the Mg, B and/or theinter-metallic compound of Mg and B are in the form of the respectivehydrides.

The dopant may be provided as a pure elemental metal. Preferably, thedopant is in the form of a dopant composition, wherein the dopantcomposition is an alcoholate, halide, hydride, organo-metallic orinter-metallic compound of one or more of the mentioned transitionmetals.

The components making up the doped hydrogen storage material may beintimately mixed in an inert atmosphere, i.e. in vacuum or in anatmosphere comprising no other reactive gaseous or vapour componentother than hydrogen. For instance, to prevent oxidation of one or moreof the components, the atmosphere should not comprise oxygen. Examplesof suitable atmospheres are for example atmospheres comprising nitrogen,hydrogen, argon gas or mixtures thereof.

A suitable method for preparing the doped hydrogen storage materialsaccording to the invention is the ‘wedge’ growth method as described inWO2005/035820, which is hereby incorporated by reference. In this methodMg, B and Ti are evaporated and deposited on a suitable substrate in anultra high vacuum system, hydrides can be prepared in hydrogencomprising atmosphere. This method allows for the preparation ofamorphous materials, whereas the method described in for instance the Liet al. reference, see hereinabove, produced well-crystallised Mg(BH₄)₂.Other suitable methods may include plasma arc methods.

The invention also relates to the use of a doped hydrogen storagematerial according to the invention to store hydrogen either reversiblyor irreversibly.

A hydrogen-comprising gas may by obtained by desorbing, or withdrawing,hydrogen from an at least partially hydrided doped hydrogen storagematerial according to the invention. In addition a partially dehydrideddoped hydrogen storage material is obtained. Depending on exactcomposition of the hydride and the desired equilibrium pressure, the atleast partially hydrided material may be dehydrided by subjecting it toa sufficiently high temperature, preferably in the range of from 20 to500° C., and a suitable pressure, preferably below the equilibriumpressure. It is an advantage of the present invention that hydrogen maybe desorbed from the doped hydrogen storage material according theinvention at significantly milder temperature conditions compared tonon-doped Mg and B based hydrogen storage materials, such as Mg(BH₄)₂.Consequently, hydrogen may be obtained from the doped hydrogen storagematerial at lower temperatures.

It has, however, been found that in order to be able to reversiblydehydride and rehydride the doped hydrogen storage material according tothe invention, a minimum amount of hydrogen must remain in the obtainedpartially dehydrided doped hydrogen storage material. The obtainedpartially dehydrided doped hydrogen storage material should stillcomprise at least 10 atomic %, in particular at least 20 atomic %, morein particular at least 30 atomic %, even more in particular at least 40atomic %, still more in particular at least 50 atomic %, of the maximumamount of atomic hydrogen which can be stored in the doped hydrogenstorage material. Reference herein to the maximum amount of hydrogenthat can be stored in the doped hydrogen storage material is to anamount of 4 hydrogen atoms per boron atom (i.e. n=4y) present in thedoped hydrogen storage material. Without wishing to be bound to aparticular theory, it is presently believed that by limiting the amountof hydrogen removed from the doped hydrogen storage material, theformation of elemental Mg and B is, at least partly, prevented. It isbelieved that the intermediate products other than elemental Mg and Bformed during dehydriding are susceptible to rehydriding.

Hydrogen is stored in the doped hydrogen storage material by contactingan at least partially dehydrided doped hydrogen storage materialaccording to the invention to a hydrogen-comprising gas, preferablyhydrogen gas, at an elevated pressure, preferably in the range of from 1to 50 bar, more preferably in the range of from 5 to 15 bar. Thetemperature at which the doped hydrogen storage material is contactedwith the hydrogen-comprising gas may be any suitable temperature,typically above 10° C., preferably in the range of from 10 to 150° C.,more preferably in the range of from 15 to 50 ° C. Suitably, the dopedhydrogen storage material is contacted with the hydrogen-comprising gasat ambient temperatures.

The doped hydrogen storage material is contacted with thehydrogen-comprising gas for any time necessary to sufficiently rehydridethe at least partially dehydrided doped hydrogen storage material.Preferably, in the range of from 1 to 24 hours, more preferably 5 to 10hours.

An at least partially rehydrided doped hydrogen storage material isobtained.

It is an advantage of the present invention that the doped hydrogenstorage material according the invention may be hydrided or rehydridedat significantly milder conditions compared to, for instance,well-crystallized Mg(BH₄)₂ as disclosed in the Li et al. reference, seehereinabove. Li et al. rehydrided a dehydrided well-crystallisedMg(BH₄)₂ by contacting the dehydrided well-crystallised Mg(BH₄)₂ withhydrogen under 400 bar, 170° C. and for 48 hours.

The doped hydrogen storage material according to the invention may beused alone or in combination with other materials to store hydrogen, forinstance in hydrogen storage tanks or hydrogen batteries.

EXAMPLES

The present invention is illustrated by the following non-limitingexamples.

Sample Preparation

Thin film samples were prepared using a ‘wedge’ growth method asdescribed in WO2005/035820.

Magnesium, boron and titanium comprising thin films were deposited onsilicon wafers (ex. Nova Electronic Materials Ltd) in a custom builtultra high vacuum system (1.3×10⁻¹² bar). The hydride samples wereprepared in a background pressure of hydrogen (ex. AirProducts, N5.5(10⁻⁶ torr)). Magnesium (ex. Alfa Aesaer, 99.98%) was evaporated at712K. Titanium (ex. Alfa Aesaer, 99.99%) was evaporated at 1832K. Bothmagnesium and titanium were evaporated from effusion cells (DCA). Boron(ex. Alfa Aesaer, 99.9%) was evaporated from an electron beam evaporator(Temescal single earth). Compositional analysis was carried out usinginductively coupled plasma mass spectroscopy.

Example 1 Material Characterization

A sample having the general formula Mg_(0.16)B_(0.16)B_(0.81)Ti_(0.03)was analysed using X-ray Diffraction (XRD). Mg(BH₄)₂ has two crystallinestructures the low temperature hexagonal structure and the hightemperature orthorhombic structure. Transition of the hexagonal to theorthorhombic crystal structure takes place at 162° C. (435 K) (see J.-H.Her, et al., Acta Cryst. B63 (2007) 561-568). To investigate thecrystallisation behaviour of the prepared doped hydrogen storagematerial according to the invention, two samples were prepared. Thefirst sample was annealed to a temperature of 150° C. (423 K) to induceformation of the hexagonal crystal structure. The second sample wasannealed to 210° C. (483 K) to induce formation of the orthorhombiccrystal structure. Both samples were analysed using XRD, Bruker D8(λ_(Cu) (1.541 nm)) integrated for 3600 s using a general area detector(GADS) over an integration range of 16.7 to 49.1 two theta. The sourcearm was setup at 11° and the detector at 25° , giving a spot size ofapproximately 1 mm².

The samples were capped with an amorphous silicon dioxide film (ca. 100nm) prior to XRD to prevent any oxidation.

For neither the low temperature or high temperature annealed sampleswere any XRD peaks identified in the XRD spectrum. This indicates thatno detectable amounts of crystalline materials were formed and theprepared material was essentially amorphous.

In addition a sample having the general formulaMg_(0.16)B_(0.81)Ti_(0.03)H_(n) was analysed as made. Also for thissample no XRD peaks were identified in the XRD spectrum. This indicatesthat no detectable amounts of crystalline materials were present and theprepared material was essentially amorphous.

Example 2 Dehydriding Measurements

Magnesium, boron, titanium thin films were deposited on arrays of microelectro mechanical (MEMS) devices to perform temperature desorptionexperiments from the thin film material libraries. Temperatureprogrammed desorption was carried out at a rate of 23 Ks⁻¹ within a highvacuum chamber (1.3×10⁻¹² bar). The hydrogen partial pressure wasmeasured using a quadrupole mass spectrometer placed 20 mm from thesample.

The hydrogen desorption behaviour of samples representative for thedoped hydrogen storage material according to the invention, i.e.0.2<x<0.6, 0.4<y<0.85, 0.005<z<0.35 and 0.15<x/y<1.5, was determinedusing the above-described method.

Table 1 shows the hydrogen onset-temperatures for a number of Ti-dopedhydrogen storage compositions. Chlopek et al.(J. Mater. Chem., 2007, 17,3496-3503), have reported for Mg(BH₄)₂, that desorption commences at290° C. All the doped hydrogen storage materials according to thepresent invention, show significantly lower hydrogen desorption onsettemperatures.

TABLE 1 Mg B Ti Mg:B Onset Temperature Sample [x] [y] [z] [—] [° C.] 10.342 0.598 0.060 0.57 243 2 0.345 0.595 0.060 0.58 216 3 0.352 0.5880.060 0.60 205 4 0.302 0.672 0.026 0.45 186 5 0.372 0.602 0.026 0.62 2126 0.336 0.616 0.048 0.55 207 7 0.354 0.598 0.048 0.59 210 8 0.289 0.6620.049 0.44 187 9 0.285 0.613 0.102 0.46 219

In order to determine the optimum composition for a storage material thegravimetric capacity was considered in addition to the onsettemperature. The obtained gravimetric capacity of a number of Ti-dopedhydrogen storage compositions is given in table 2. The highest hydrogenstorage capacity was obtained for a Ti-doped hydrogen storage materialhaving a composition of B_(0.58)Mg_(0.36)Ti_(0.06). This material showeda hydrogen desorption onset-temperature of 250° C., with a peak in thehydrogen desorption observed at 425° C. using a heating rate of 23 K/s.

TABLE 2 Mg B Ti Hydrogen storage Sample [x] [y] [z] capacity [wt %] 100.497 0.411 0.092 5.2 11 0.441 0.470 0.0989 5.2 12 0.352 0.588 0.09605.3 13 0.343 0.608 0.0949 5.6 14 0.437 0.471 0.0992 6.3 15 0.354 0.5980.0949 6.3 16 0.336 0.616 0.0949 6.4 18 0.309 0.639 0.0952 7.1 18 0.3650.543 0.0993 7.1 19 0.350 0.594 0.0956 9.9 20 0.365 0.578 0.0957 10.6

Example 3 Dehydriding and Rehydriding Experiments

A sample as characterised in Example 1 was hydrided until no furtheruptake of hydrogen was observed. The hydride sample was dehydrided in asecond step by a temperature programmed desorption carried out at a rateof 23 Ks⁻¹ within a high vacuum chamber. The hydrogen partial pressurewas measured using a quadrupole mass spectrometer placed 20 mm from thesample. Dehydriding was discontinued at a temperature of 595° C. andcooled to room temperature at a rate of 23 Ks⁻¹. At this stage thehydrogen storage material still comprised approximately 50 atomic % ofthe maximum amount of atomic hydrogen that could be stored. The maximumamount of atomic hydrogen was determined by thermal desorption analysis.

The obtained partially dehydrided sample was rehydrided in a third stepby contacting the sample with hydrogen gas at a pressure of 10 bar for 8hours under ambient temperature conditions (23° C.) No formation of MgH₂was observed.

In a final step, the at least partially rehydrided sample obtained fromthe third step was again dehydrided following the same temperatureprogram used for the initial sample to determine the amount of hydrogenwhich could be reversibly reabsorbed. It was observed the 23% of theamount of hydrogen desorbed in the second step from the initial samplewas reversibly reabsorbed into the partially dehydrided doped hydrogenstorage material.

This shows that the doped hydrogen storage material according to thepresent invention can reversibly store hydrogen.

1. Doped hydrogen storage material according to the general formula:Mg_(x) B_(y) M_(z) H_(n) wherein: (i) the ratio of x/y is in the rangeof from 0.48 to 0.70, but outside the range of from 0.49 to 0.51; (ii) zis in the range of from 0.005 to 0.35; (iii) x+y+z equals 1; (iv) M isat least one metal selected from the group consisting of Sc, Ti, V, Cr,Mn, Fe, Co, Ni, Cu and Zn; (v) n is no more than 4y; and wherein atleast part of the doped hydrogen storage material is amorphous.
 2. Dopedhydrogen storage material according to claim 1, wherein at least 90% ,of the doped hydrogen storage material is amorphous.
 3. Doped hydrogenstorage material according to claim 1, wherein z is in the range of from0.005 to 0.1.
 4. Doped hydrogen storage material according to claim 1,wherein M is Ti.
 5. Doped hydrogen storage material according to claim1, wherein y is in the range of 0.5 to 0.85. 6-9. (canceled)
 10. Dopedhydrogen storage material according to claim 1, wherein M is Ni. 11.Doped hydrogen storage material according to claim 1, wherein M is Tiand Ni.
 12. Doped hydrogen storage material according to claim 1,wherein the ratio of x/y is in the range of from 0.51 to 0.70.